FIG. 3 is a diagram to show the configuration of an electric power conversion apparatus in a related art described in JP-A-10-174444, for example. In the figure, numeral 20 denotes an AC power supply, numeral 21 denotes a converter section for converting AC power supplied from the AC power supply 20 into DC power, numerals 22a, 22b, 22c, and 22d denote semiconductor switches making up a single-phase bridge circuit as the converter section 21, and numerals 23a, 23b, 23c, and 23d denote diodes connected in inversely parallel to the semiconductor switches 22a, 22b, 22c, and 22d. Numeral 24 denotes a smoothing capacitor connected to a DC side of the converter section 21, and numeral 25 denotes a load. Numeral 26 denotes a filter capacitor and numeral 27 denotes a reactance component of primary wiring for connecting the AC power supply 20 and the electric power conversion apparatus.
Numeral 31 denotes a reactor connected to an AC side of the converter section 21, numeral 32 denotes an AC voltage detector for detecting AC voltage of the AC power supply 20, numeral 33 denotes an AC detector for detecting AC of the AC power supply 20, and numeral 34 denotes a DC voltage detector being connected to both ends of the smoothing capacitor 24, for detecting DC voltage of the smoothing capacitor 24.
The main circuitry of the electric power conversion apparatus in the related art is made up of the converter section 21, the smoothing capacitor 24, the reactor 31, the AC voltage detector 32, the AC detector 33, and the DC voltage detector 34.
Numeral 40 denotes a control unit of the electric power conversion apparatus. The control unit 40 is made up of a DC voltage control section 41, a reference sine wave generation section 42, an AC command value calculation means 43, AC control section 44, a pulse width modulation section 45, and a switch drive section 46.
The AC control section 44 is made up of a subtraction section 47, a proportional calculation section 48, series connection of a band-pass filter (BPF) 49 and a proportional calculation section 50, which are placed in parallel with the proportional calculation section 48, and an addition section 51, to control in a discrete system. The BPF 49 is a band-pass filter with power supply frequency fs as the center frequency for allowing only the power supply frequency component to pass through and removing other frequency components.
The operation of the control unit 40 of the electric power conversion apparatus in the related art will be discussed with reference to FIG. 3.
The DC voltage control section 41 performs proportional integration calculation of deviation between DC voltage command value Vd* set by an output voltage setting device (not shown) and DC voltage detection value Vd# detected by the DC voltage detector 34 as DC voltage supplied to the load 25, and outputs the result as AC amplitude command value 1s*. The reference sine wave generation section 42 outputs reference sine wave sincot having the same period and the same phase as AC voltage detection value Vs# output from the AC voltage detector 32.
The AC command value calculation section 43 multiples the AC amplitude command value 1s* output from the DC voltage control section 41 by the reference sine wave sincot output from the reference sine a wave generation section 42, and outputs the result as AC command value is*.
The AC control section 44 inputs the AC command value is* output from the AC command value calculation section 43 and AC detection value is# detected by the AC detector 33 as AC of the AC power supply 20, and outputs AC voltage command value Vc*.
The pulse width modulation section 45 generates and outputs an on/off signal for performing on/off control of the semiconductor switch 22a, 22b, 22c, 22d making up the converter section 21 based on the command of the AC voltage command value Vc* output from the AC control section 44.
The switch drive section 46 turns on/off the semiconductor switch 22a, 22b, 22c, 22d according to the on/off signal output from the pulse width modulation section 45, thereby converting the AC power supplied from the AC power supply 20 into DC power.
Next, an operation of the AC control section 44 inputting the AC command value is* and the AC detection value is# and outputting the AC voltage command value Vc* will be discussed.
In the AC control section 44, the subtraction section 47 calculates deviation ei between the AC command value is* output from the AC command value calculation section 43 and the AC detection value is# detected by the AC detector 33.
The proportional calculation section 48 with proportional gain Kp multiplies the deviation ei by Kp. The deviation ei is input to the series connection of the BPF 49 and the proportional calculation section 50, which are placed in parallel with the proportional calculation section 48, and the proportional calculation section 50 with proportional gain Kps multiplies a deviation of the deviation ei in the vicinity of the power supply frequency passing through the BPF 49 by Kps.
The addition section 51 adds an output signal provided by multiplying the deviation ei by Kp by the proportional calculation section 48 and an output signal provided by multiplying the deviation of the deviation ei in the vicinity of the power supply frequency passing through the BPF 49 by Kps by the proportional calculation section 50 to obtain the AC voltage command value Vc*.
In the electric power conversion apparatus in the related art, the control unit 40 calculates the AC voltage command value Vc* from the DC voltage command value Vd*, the DC voltage detection value Vd* detected by the DC voltage detector 34 as the DC voltage supplied to the load 25, and the AC detection value is# detected by the AC detector 33 as the AC of the AC power supply 20 as described above and controls the main circuitry of the electric power conversion apparatus, thereby matching the DC voltage and the AC of the electric power conversion apparatus with the DC voltage command value and AC command value, respectively.
FIG. 4 shows the waveform of the AC command value is* output from the AC command value calculation section 43 in the control unit 40 of the electric power conversion apparatus in the related art.
FIG. 5 shows the waveform of the AC detection value is# detected by the AC detector 33, in the control unit 40 of the electric power conversion apparatus in the related art, in which high-order harmonic content superposed on the input current waveform. In the figure, (a) shows the waveform at the normal operation time and (b) shows the waveform when resonance occurs.
A comparison is made between the waveform of the AC command value is* input to the subtraction section 47 of the AC control section 44 and the waveform of the AC detection value is# according to FIGS. 4 and 5.
Microscopically, the waveform of the AC command value is* output from the AC command value calculation section 43 based on the discrete system control becomes a stepwise waveform changing at every given sampling time as shown in FIG. 4. In contrast, the waveform of the AC detection value is# detected by the AC detector 33 becomes a continuous waveform with higher response speed than the sampling time of the AC command value is* shown in FIG. 4, as shown in FIG. 5(a).
Thus, in the deviation ei between the AC command value is* and the AC detection value is#, calculated by the subtraction section 47, waveform distortion equivalent to the sampling time in the AC command value calculation section 43 based on the discrete system control occurs in the input current waveform.
In the electric power conversion apparatus in the related art, if the primary wiring connecting the AC power supply 20 and the electric power conversion apparatus becomes long and the sampling time in the AC command value calculation section 43 based on the discrete system control and the time constants of the reactance component 27 of the primary wiring and the filter capacitor 26 become equal, even if slight distortion of the input current waveform occurs, the high-order harmonic content generated due to resonance of the inductance component of the primary wiring and the capacitor connected as filter is superposed on the input current waveform and the input current waveform is distorted as shown in FIG. 5(b).
The AC control section 44 for control in the discrete system in the related art uses the BPF 49 with the power supply frequency fs as the center frequency. In the configuration wherein the proportional gain in the vicinity of the power supply frequency fs of the discrete system control is increased equivalently by connecting the series connection of the BPF 49 and the proportional calculation section 50 in parallel with the proportional calculation section 48, two paths of the path of the proportional calculation section 48 and the path of the series connection of the BPF 49 and the proportional calculation section 50 are adopted and therefore the harmonic components generated due to the resonance of the inductance component of the primary wiring and the capacitor connected as filter cannot be removed.
Thus, the electric power conversion apparatus in the related art involves a problem of limiting the length of the primary wiring to avoid the resonance of the inductance component of the primary wiring and the capacitor connected as filter.
The invention is intended for solving the problems as described above and it is an object of the invention to provide a control unit of an electric power conversion apparatus that can output stable DC voltage even if the sampling time based on discrete value control and the time constants of the inductance component of the primary wiring and a capacitor connected as a filter become equal.